US3185894A - Means for indicating faults in the crucible of induction melting furnaces - Google Patents

Description

May 25, 1955 BECKIUS 3,185,394

MEANS FOR INDICATING FAULTS IN THE CRUCIBLE 0F INDUCTION MELTING FURNACES Filed Aug. 10, 1961 IN VEN TOR. Iv r 35 c )CI I Maw/g United States Patent 6 Claims: (Cl. 317-9 Hitherto known systems for the supervision of the condition of the crucible for induction melting furnaces are based upon the measuring of the leakage current passing from the furnace coil and through the melt to ground. In order to bring about such a current, either a separate voltage source is used, in which case the leakage current is relatively independent of which part of the crucible has undergone deterioration in its condition, or the operational voltage of the furnace is used. In both cases a connection lead, which is always required for continuous supervision, has to be traced between the furnace circuit and ground. The leakage current, however, in the latter case will be dependent on the potential of that part of the furnace coil in the vicinity of which the deterioration of the crucible has taken place. The first mentioned system therefore is deemed more advantageous, and the present invention relates to such a system.

It is usual to indicate the condition of the crucible by direct measuring of the leakage current. If a fault on the crucible has become so consider-able that direct short-circuiting occurs between the melt and the furnace coil, or if the resistance of the crucible is decreased considerably, the risk occurs at continuous supervision that the indicating means becomes overloaded or destroyed, since it is dimensioned for the measuring of very feeble currents.

The grounding of the furnace circuit implies that the melt, due to a fault of the crucible, is under tension and gets a potential against ground, for although the grounding is brought about by the melt it is not completely certain whether this ground connection exists or not. The furnace operator handling the furnace and who, by means of rods or the like, may come into contact with the coil, the melt or the crucible wall may therefore be exposed to danger of coming into contact with the tension. Particular care must be taken if comparatively high voltages and low frequencies in the furnace circuit are concerned. Therefore one has, as a rule, not employed crucible fault indication means for continuous supervision, but carried out the furnace circuits without grounding in order to minimize the danger of contact with the melt, or ground circuits have been intermittently coupled in for short con tacts of the coil insulation or the like.

The present invention relates to a crucible fault indicating means fed with a separate auxiliary voltage such as from an alternating current network, into an indicating circuit. The means is based on the fact that the increased leakage current which occurs at the occurrence of a fault of the crucible, bring about a voltage drop in a resistance series connected in the auxiliary voltage circuit. The auxiliary circuit comprises one Winding of an insulating transformer, the other winding of which is inter-connected between ground and a point of the furnace circuit.

The feature of the invention resides in that between said winding and said point there is inter-connected an electric blocking circuit which is tuned to the frequency of the furnace circuit and is so dimensioned that the current passing through the blocking circuit is maintained below a value dangerous for a human being, usually below 10 ma.

The condition of the crucible is indicated by measuring 3,185,894 Patented May 25, 1965 the said voltage drop across the said resistance. This voltage drop may also be used for interrupting the furnace circuit if required. When there is no insulating fault between the coil and the melt no current flows in the auxiliary voltage circuit and the voltage relations in said circuit are suitable, for example a certain voltage is present across the primary winding. When an insulation fault between the coil and the melt occurs, a current flows in the auxiliary circuit and a certain voltage drop takes place over said series resistance. The voltage relations in the circuit are thus changed and, for example, the voltage across the primary winding is changed. Said changed voltage may be indicated in different ways, for example, by means of relays utilized for switching-off the power circuit. The indicating device of the fault indicating circuit, which may be a voltage meter, is not overloaded even in the event that a stronger current occurs, for example, at direct short-circuiting between the melt and the furnace coil.

The blocking circuit is tuned to the frequency of the power circuit and so dimensioned that, at such frequencies and frequencies close to this frequency the current passing through the blocking circuit is maintained below the value which is dangerous when passing through a human body. A current not stronger than 10 me. pass ing through the human body when in contact with an electric mains may be considered as not dangerous. The blocking circuit has not only the purpose of eliminating the danger for the personnel when the furnace is under operation but also of preventing capacitively transmitted voltage of furnace frequency disturbing the indication of i the condition of the crucible.

There must be a certain difference between the frequency of the furnace circuit and the frequency of the auxiliary voltage in order to give the means the required sensitivity. If the ratio between the frequencies of the furnace circuit and of the auxiliary voltage circuit is less than 40:1, a rectifier is used in series with the blocking circuit for transforming the alternating auxiliary indicating current into direct current before passing through the blocking circuit and the resistance of the crucible. The auxiliary current circuit is thereby influenced to a minor degree by the impedance of the blocking circuit.

In furnace constructions where the conductors of the furnace coil are completely surrounded or are covered on the side directed against the melt by an electrically conductive shield which i not in electrically conducting contact with either the furnace coil or the melt, the shield is connected to one pole of the blocking circuit whereby capacitive transmission of disturbances from the furnace to the indicating circuit can be eliminated.

The means according to the invention will be described with reference to the accompanying drawings in which FIGURES 1, 2, 3 and 4 illustrate the above examples of carrying out the invention.

A coreless furnace with a crucible 3, surrounded by a coil 2 is fed from an AC. source, which, in FIG. 1, is of high frequency. In such furnaces there is a need to detect insulation failures between the coil 2 and the melt. Therefore at the crucible 3 a ground connection 8 is arranged. Circuit 1 in FIGURE 1 is consequently a power circuit. From one of the power lines a connection to the ground connection through the secondary winding of an insulation transformer 7 is arranged. The primary winding of this transformer is connected to a low frequency auxiliary network (at 6) in series with a resistance 18. The voltage over the primary winding of the transformer 7 is measured at It) and indicated in parallel at 9 by means of a relay, a signal device, or the like. The secondary circuit of transformer 7 is connected over a very high resistance to the melt when the coil is satisfactorily insulated. In this case almost no current is fed to the i11- dioating circuit over transformer 7 and thus the voltage drop over resistance 18 is negligible, as there is no current in the primary winding of the auxiliary circuit. If the insulation breaks down, the resistance between the coil and the melt decreases and a current from the transformer flows in the indication circuit secondary winding of transformer 7power line-coil 2melt-ground S secondary winding at 7. At the same time a current flows in the primary winding of transformer 7 through resistance 18 and thus a voltage drop takes place across resistance 18 and the changed voltage relations are measured at 10 and 9. The relay at 9 is thus actuated and the power in circuit 1 is switched off (not shown). The circuit 7123-8-7 is called a grounded error protective circuit or indication circuit and protects the furnace from the consequences of insulation faults between the coil and the melt.

There is, however, a drawback connected with such a protective circuit with ground connection. Suppose a person by means of a metallic rod or the like contacts the melt or coil when an insulation fault occurs and conductor S has been broken. In this case a current from the A.C. source at 1 runs through such person to ground and from ground through the secondary winding at 7 to the power line at 1, and said current may be dangerous. In order to protect said person from the risk of a broken ground connection (at 8), a secondary protective means or blocking circuit for limiting current from the source in circuit 1 and at the frequency of said source is inserted in aid protective circuit. The blocking circuit comprises a condenser 11, a reactance coil 12, and an indicating device 13 for indicating the state of the tuning of the blocking circuit. Said secondary protective means or blocking circuit is tuned in relation to the frequency in the circuit 1 in such a manner that no current through said blocking circuit can exceed dangerous values, such as ma. Current from circuit 4 over transformer 7 is not blocked through said blocking device but such current of appreciable degree is only present where insulation faults between the coil and the melt occur and then only for a short moment before the power in circuit 1 is switched off.

According to the example shown in FIGURE 2 it is assumed that the furnace circuit is supplied with commercial network frequency. The auxiliary voltage supplied to the indicating circuit and taken from the network is rectified by a rectifier means 14 inter-connected on the secondary winding of the insulating transformer 7, and provided with a spark gap device 15 which indicate any diminished voltage drops over the rectifier bridge which may possibly occur during operation. In FIG. 2 the frequency of the power circuit 1 is network frequency (60 or 50 cycles) and in auxiliary circuit 4 network fre- I quency, rectified through bridge 14. In this case the blocking means is tuned for blocking network frequency, but DC. from bridge 14 is not blocked. FIGURE 3 shows an embodiment wherein a voltage divider 16 is inter-connected between the furnace circuit and the blocking circuit. The said point of the voltage divider is connected to the blocking circuit while the terminals are connected to the supply leads of the furnace coil 2. By this arrangement more favourable dimensioning conditions for the blocking circuit are obtained.

According to FIGURE 4, an electrically conducting shield 17 is interposed between the furnace coil and the crucible. The blocking circuit is connected to said shield, since this controls the insulation and homogenity of the crucible wall, and there must be a large resistance be- The embodiments described may be used in combi A nation.

Although the means according to the invention has been described for use in an induction furnace for melting purposes it may also be employed in combination with other induction furnaces such as furnaces for warm ing purposes, wherein the charge is surrounded by a coil with insulated or not insulated conductors and where contact between the conductors and the charge is to be indicated.

I claim:

1. In combination with a crucible having an induction coil adjacent thereto and an A.C. power supply connected to the coil for feeding current thereto, a connection from the crucible to ground, an indicator circuit for indicating an insulation fault in the crucible comprising a transformer having a secondary winding connected between said ground connection and said power supply, an auxili-ary A.C. source, means including a resistance connecting said auxiliary source to the primary winding of said transformer, measuring means in said connecting means, said measuring means being responsive to the flow of current occurring in said connecting means upon a break-down in insulation between the coil and the crucible, and blocking means coupled in series with the secondary winding of the transformer, said blocking means being tuned to the frequency of the power supply and including means to keep the flow of current therethrough at such frequency relatively low without substantially impeding flow of current in said connecting means.

2. A device according to claim 1, in which the blocking means comprises a capacitor in parallel with a reactor, said blocking means being coupled in series between one terminal of the secondary winding of said transformer and said power supply.

3. In a device according to claim 1, rectifier means between the secondary of the transformer and the power supply and ground for rectifying current fed from said auxiliary A.C. source into said transformer.

4. Means according toclaim 1, wherein the ratio between the frequencies of the power supply circuit and of the connecting means is small, and the blocking means includes a rectifier connected to the secondary transformer.

5. Means according to claim 1, in which the blocking means is connected to the power supply by a voltage divider, the mid point of which is connected to the blocking means, while its terminals in parallel are connected to the power supply.

6. Means according to claim 1, in which the furnace coil is provided with an electrically conducting shield insulated from the conductors of the furnace coil and from the melt and connected to the blocking means.

References Cited by the Examiner UNITED STATES PATENTS 1,778,398 10/30 Northrup 317-9 X 1,801,790 4/ 31 Breisky .et al. 340242 X 2,474,711 6/49 Yonkers et al. 317-9 X 2,614,200 10/52 McNair 317-9 X 2,990,542 6/61 Seitz -Q. 1327 X SAMUEL BERNSTEIN, Primary Examiner. LLOYD McCOLLUM, Examiner.

Claims (1)

1. IN COMBINATION WITH A CRUCIBLE HAVING AN INDUCTION COIL ADJACENT THERETO AND AN A.C. POWER SUPPLY CONNECTED TO THE COIL FOR FEEDING CURRENT THERETO, A CONNECTION FROM THE CRUCIBLE TO GROUND, AN INDICATOR CIRCUIT FOR INDICATING AN INSULATION FAULT IN THE CRUCIBLE COMPRISING A TRANSFORMER HAVING A SECONDARY WINDING CONNECTED BETWEEN SAID GROUND CONNECTION AND SAID POWER SUPPLY, AN AUXILIARY A.C. SOURCE, MEANS INCLUDING A RESISTANCE CONNECTING SAID AUXILIARY SOURCE TO THE PRIMARY WINDING OF SAID TRANSFORMER, MEASURING MEANS IN SAID CONNECTING MEANS, SAID MEASURING MEANS BEING RESPONSIVE TO THE FLOW OF CURRENT

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